Small crystal ZSM-5, its synthesis and use

ABSTRACT

A synthetic porous crystalline material has the structure of ZSM-5 and a composition involving the molar relationship: 
     
       
         X 2 O 3 :( n )YO 2 , 
       
     
     wherein X is a trivalent element; Y is a tetravalent element; and n is less than 25, and wherein the slope of the nitrogen sorption isotherm of the material at a partial pressure of nitrogen of 0.4 to 0.7 and a temperature 77° K is greater than 30. The material has a mesoporous surface area (MSA) greater than 45 m 2 /g and is useful as a catalyst in the liquid phase isomerization of xylene.

FIELD OF THE INVENTION

This invention relates to small crystal ZSM-5, its synthesis and its usein catalytic processes, particularly xylene isomerization.

BACKGROUND OF THE INVENTION

ZSM-5 and its synthesis using tetrapropylammonium (TPA) cations as adirecting agent are disclosed in U.S. Pat. No. 3,702,886. U.S. Pat. No.3,926,782 discloses hydrocarbon conversion over ZSM-5 crystals having acrystal size of 0.005-0.1 micron synthesized in the presence of TPAcations.

U.S. Pat. No. 4,151,189 discloses that ZSM-5 can be synthesized in thepresence of a primary amine having 2-9 carbon atoms, particularlyn-propylamine. U.S. Pat. No. 5,369,071 discloses the use ofn-propylamine in the synthesis of ZSM-5 with a silica to alumina ratioas low as 20.3 from a reaction mixture having a pH 10-14, an OH−/SiO₂ratio of 0.1-0.3, an M/SiO₂ ratio of 0.2-0.6 (where M is an alkali oralkaline earth metal) and an H₂O/SiO₂ ratio of 10-35.

EP-A-106552 teaches that ZSM-5 and ferrierite can be synthesized in theabsence of an inorganic directing agent by using an amorphous granularsilica-alumina as the source of silicon and aluminum. The resultantZSM-5 is said to have a silica to alumina molar ratio of 15-100 but theonly ZSM-5 product exemplified has a silica to alumina molar ratio of58.8. EP-A-106552 fails to disclose the crystal size of the ZSM-5produced.

EP-A-306238 discloses that ZSM-5 crystals having a platelet morphologywith two dimensions of at least 0.05 micron, typically at least 0.1micron, and a third dimension less than 0.02 micron can be synthesizedfrom a non-organic synthesis mixture having at least 35 wt % solids andan OH−/SiO₂ ratio of at least 0.11.

Other non-organic synthesis routes for ZSM-5 are known and commerciallypracticed and typically produce a material having a silica to aluminamolar ratio of 20-30 and a crystal size of about 0.2‥0.5 micron.

To date it has proved extremely difficult to produce ZSM-5 from reactionmixtures with silica to alumina molar ratios less than about 20, whichcould produce crystals with correspondingly lower framework silica toalumina molar ratio. Framework aluminum sites are responsible for theacid activity of zeolites, and it is desirable for many catalytic usesto be able to produce ZSM-5 with a framework silica to alumina molarratio as low as possible. Similarly, for catalytic uses where rapiddiffusion of reactants and products into and out of the zeolite isdesirable, it is important to be able to produce ZSM-5 with a smallcrystal size, for example less than 0.1 micron.

The problem of producing ZSM-5 with a low silica to alumina molar ratiohas been particularly pronounced in the case of small crystal materials.Thus to date small crystal ZSM-5, with a crystal size of less than 0.1micron, has been obtained only with silica/alumina ratios higher thanapproximately 23:1.

The crystal size of a zeolite can be determined by direct measurementusing electron microscopy. However, other indirect methods ofdetermining crystal size are available and can be useful indifferentiating between small crystal materials, especially when noexact size can be assigned visually as the result of sizepolydispersity, irregular/non-uniform shape and/or extensive crystalintergrowth. For example, the nitrogen adsorption/desorption isothermshowing the amount of nitrogen adsorbed by a solid at 77° K as thefunction of relative partioal pressure p/p₀ can be used to gauge andcompare average crystal size of materials. The isotherm can be used tocalculate apparent internal (zeolite) and external (mesoporous) surfacearea, ZSA and MSA, respectively, of the crystals. Increasing MSAindicates decreasing crystal size. At low nitrogen partial pressures theisotherm tracks filling of the zeolite micropores but at higher relativepartial pressures, i.e. 0.4-0.7 for ZSM-5, the slope of the isothermreflects the crystal size. This latter approach is useful when ambiguityin determining the MSA/ZSA split may arise.

According to the invention, a novel form of ZSM-5 has now been producedwith a combination of an unusually low silica to alumina molar ratio anda very small crystal size.

It is to be appreciated that, although ZSM-5 is normally synthesized asan aluminosilicate, the framework aluminum can be partially orcompletely replaced by other trivalent elements, such as boron, ironand/or gallium, and the framework silicon can be partially or completelyreplaced by other tetravalent elements such as germanium.

SUMMARY OF THE INVENTION

In one aspect, the invention resides in a synthetic porous crystallinematerial having the structure of ZSM-5 and a composition involving themolar relationship:

X₂O₃:(n)YO₂,

wherein X is a trivalent element, such as aluminum, boron, iron and/orgallium, preferably aluminum; Y is a tetravalent element such as siliconand/or germanium, preferably silicon; and n is less than 25, and whereinthe slope of the nitrogen sorption isotherm of the material at a partialpressure of nitrogen of 0.4 to 0.7 and a temperature of 77° K is greaterthan 30.

Preferably, the slope of the nitrogen sorption isotherm at said partialpressure of nitrogen of 0.4 to 0.7 and said temperature of 77° K isgreater than 50.

Preferably, n is about 15 to about 20.

Preferably, the crystalline material has an alpha value in excess of1300.

Preferably, the crystalline material has a BET surface area in excess of400 m²/g in which the MSA (mesoporous surface area) is greater than 45m²/g and the ratio of the ZSA (zeolite surface area) to MSA is less than7.

In a further aspect, the invention resides in a hydrocarbon conversionprocess employing a catalyst comprising the synthetic porous crystallinematerial of said one aspect of the invention.

Preferably, the hydrocarbon conversion process is xylene isomerizationor toluene disproportionation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the nitrogen adsorption isotherms for ZSM-5 producedaccording to Example 1 and for a variety of conventional ZSM-5 materialsof different crystal size labelled Catalysts 1-4.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention provides a novel form of high activity, smallcrystal ZSM-5 having a composition involving the molar relationship:

X₂O₃:(n)YO₂,

wherein X is a trivalent element, such as aluminum, boron, iron and/orgallium, preferably aluminum; Y is a tetravalent element such as siliconand/or germanium, preferably silicon; and n is less than about 25,preferably 15-20.

The small crystal size of the novel ZSM-5 of the invention is manifestedin a unique nitrogen sorption isotherm wherein the slope of the isothermat a partial pressure of nitrogen of 0.4 to 0.7 and a temperature of 77°K is greater than 30. In contrast, the slope of the nitrogen adsorptionisotherm of conventional ZSM-5 materials within the same partialpressure range and at the same temperature varies from <10 for crystalsizes >1micron to 10-15 for crystal sizes between 0.2-0.5 micron and20-25 for conventional small crystal materials having a size less than0.1 micron.

Measurement of nitrogen sorption isotherms and of BET, MSA and ZSAsurface areas is well known in the art, e.g. as ASTM Method D 4365-95“Standard Test Method for Determining Micropore Volume and Zeolite Areaof Catalysts”.

The small crystal ZSM-5 of the invention also exhibits a BET surfacearea in excess of 400 m²/g in which the MSA (mesoporous surface area) isgreater than 45 m²/g, preferably greater than 75 and more preferably atleast 100, and in which the ratio of the ZSA (zeolite surface area) toMSA is less than 8, and more preferably is less than 5. In contrast, theMSA for conventional ZSM-5 materials varies between 10 and 40 m²/g andthe ratio of the ZSA to MSA varies between 9 and 40.

The small crystal size of the ZSM-5 of the invention can also be deducedfrom other sorption measurements, for example by measuring the rate ofsorption of 2,2-dimethylbenzene at 120° C. and 60 torr (8 kPa)hydrocarbon pressure. Based on such sorption measurements, the DiffusionParameter, D/r² wherein D is the diffusion coefficient (cm²/sec) and ris the crystal radius (cm), can be derived provided the assumption ismade that the plane sheet model describes the diffusion process. Thusfor a given sorbate loading Q, the value Q/Q_(∞), where Q_(∞) is theequilibrium sorbate loading, is mathematically related to (Dt/r²)^(1/2)where t is the time (sec) required to reach the sorbate loading Q.Graphical solutions for the plane sheet model are given by J. Crank in“The Mathematics of Diffusion”, Oxford University Press, Ely House,London, 1967. The small crystal ZSM-5 of the invention will normallyhave a Diffusion Parameter, D/r², of at least 2000×10⁻⁶ and preferablyat least 2500×10⁻⁶.

The small crystal ZSM-5 of the invention will normally be comprised ofcrystals of which at least 50% by weight have dimensions less than 0.05micron as measured by transmission electron microscopy (TEM).

Since the small crystal ZSM-5 of the invention has a uniquely lowsilica/alumina molar ratio (that is high aluminum content), the hydrogenform of the material has an extremely high catalytic activity. Catalyticactivity of zeolites, such as ZSM-5, is typically measured by AlphaValue, which compares the catalytic cracking activity of the catalyst(rate of normal hexane conversion per volume of catalyst per unit time)with the activity of a standard silica-alumina cracking catalyst. TheAlpha Test is described in U.S. Pat. No. 3,354,078; in the Journal ofCatalysis, Vol. 4, p. 527 (1965); Vol. 6, p. 278 (1966); and Vol. 61, p.395 (1980), the entire contents of which are incorporated herein byreference. The experimental conditions of the test used herein include aconstant temperature of 538° C. and a variable flow rate as described indetail in the Journal of Catalysis, Vol. 61, p. 395. In the hydrogenform, the small crystal ZSM-5 of the invention has an Alpha Value inexcess of 1000 and preferably in excess of 1300.

Small crystal high activity ZSM-5 of the invention is synthesized usingan amorphous silica-alumina having a silica/alumina molar ratio of 10:1to 25:1 as a raw material either with or without additional silica andalumina sources. Suitable amorphous silica-alumina materials areconveniently prepared by co-precipitation from soluble silica andalumina sources followed by suitable washing and cation exchange with anammonium salt. They are also available from commercial suppliers likeWRGrace. The reaction mixture may be totally inorganic but morepreferably contains n-propylamine as an organic directing agent (R).More specifically the reaction mixture has a composition, expressed interms of mole ratios of oxides, as follows:

Component Useful Preferred SiO₂/Al₂O₃ 10-25 15-20 H₂O/SiO₂ 5-30 8-20R/SiO₂ 0-1.0 0-0.4 M/SiO₂ 0.01-0.5 0.15-0.2

where M is an alkali or alkaline earth metal.

The synthesis method of the invention functions with or without addednucleating seeds. In a preferred embodiment, the reaction mixturecontains 0.05-5 wt % nucleating seeds.

Crystallization is carried out under either stirred or static conditionsat a temperature of 100 to 200° C., preferably 120 to 170° C., for 6hours to 10 days and the resultant ZSM-5 crystals are separated from themother liquor and recovered.

When used as a catalyst, it may be desirable to incorporate the smallcrystal ZSM-5 of the invention with another material resistant to thetemperatures and other conditions employed in organic conversionprocesses. Such materials include active and inactive materials andsynthetic or naturally occurring zeolites as well as inorganic materialssuch as clays, silica and/or metal oxides such as alumina. The lattermay be either naturally occurring or in the form of gelatinousprecipitates or gels including mixtures of silica and metal oxides. Useof a material which is active, tends to change the conversion and/orselectivity of the catalyst in certain organic conversion processes.Inactive materials suitably serve as diluents to control the amount ofconversion in a given process so that products can be obtainedeconomically and orderly without employing other means for controllingthe rate of reaction. These materials may be incorporated into naturallyoccurring clays, e.g., bentonite and kaolin, to improve the crushstrength of the catalyst under commercial operating conditions. Saidmaterials, i.e., clays, oxides, etc., function as binders for thecatalyst. It is desirable to provide a catalyst having good crushstrength because in commercial use it is desirable to prevent thecatalyst from breaking down into powder-like materials. These clayand/or oxide binders have been employed normally only for the purpose ofimproving the crush strength of the catalyst.

Naturally occurring clays which can be composited with the new crystalinclude the montmorillonite and kaolin family, which families includethe subbentonites, and the kaolins commonly known as Dixie, McNamee,Georgia and Florida clays or others in which the main mineralconstituent is halloysite, kaolinite, dickite, nacrite, or anauxite.Such clays can be used in the raw state as originally mined or initiallysubjected to calcination, acid treatment or chemical modification.

In addition to the foregoing materials, the ZSM-5 can be composited witha porous matrix material such as silica-alumina, silica-magnesia,silica-zirconia, silica-thoria, silica-beryllia, silica-titania as wellas ternary compositions such as silica-alumina-thoria,silica-alumina-zirconia silica-alumina-magnesia andsilica-magnesia-zirconia.

The relative proportions of finely divided ZSM-5 material and inorganicoxide matrix vary widely, with the ZSM-5 content ranging from about 1 toabout 90% by weight and more usually, particularly when the composite isprepared in the form of beads, in the range of about 2 to about 80 wt. %of the composite.

The small crystal ZSM-5 of the invention is useful as a catalyst inhydrocarbon conversion reactions where high activity and/or rapiddiffusion of reactants and/or products out of the zeolite pores isimportant. For example, the ZSM-5 of the invention is useful as acatalyst for disproportionation of toluene and for the liquid orsupercritical phase isomerization of xylenes under conditions includinga temperature of from about 300 to 600° C., a pressure of from about 1to 100 atmospheres (about 100 to 10,000 kPa), a weight hourly spacevelocity (WHSV) of about 0.5 to 100 hr⁻¹, and a hydrogen/hydrocarbonmole ratio of 0 (no added hydrogen) to about 10. Where the catalyst isused in xylene isomerization, the xylene isomerization step could be thesecond stage in a two stage process in which a C₈ aromatics stream wasinitially subjected to an ethylbenzene conversion step.

In order to more fully illustrate the nature of the invention and themanner of practicing same, the following examples are presented.

EXAMPLE 1

A synthesis mixture was produced from 320 g of water, 37.5 g ofamorphous silica-alumina having silica/alumina molar ratio of 10:1 (assupplied by WRGrace), 24 g of HiSil 233, 10 g of 50% NaOH solution, 7 gof N-propylamine and 3 g of ZSM-5 seeds. The mixture was reacted at 154°C. (310° F.) in a Parr autoclave with stirring at 200 RPM. Thecrystallization was interrupted after 24 and 48 hr. for sampling, and 2g of 50% NaOH was added following the latter. The product obtained after72 hr. synthesis was filtered, washed with water and dried at 100° C.The solid was identified as ZSM-5 with a small amount of crystallineimpurity using powder X-ray diffraction.

The as-synthesized material was converted into the hydrogen form bymultiple ion exchange with ammonium chloride solution at 82° C. (180°F.), followed by drying in an oven and calcination at 540° C. in air for6 hours. The resultant HZSM-5 had a silica/alumina molar ratio of about15:1, an Alpha Value of 1365, and a BET surface area of 415 m²/g (290zeolite surface area and 125 matrix surface area). The small crystalsize of the material was demonstrated by TEM (which indicated a crystalsize <0.05 μ) and confirmed by a high D/r² parameter for2,2-dimethylbenzene of about 3700×10⁻⁶.

The nitrogen adsoption isotherm of the HZSM-5 of Example 1 is plotted inFIG. 1 against those of the following conventional ZSM-5 materials:

1. ZSM-5 with an average crystal size of 2 microns

2. Non-organic ZSM-5 with a crystal size of 0.2-0.5 microns.

3. ZSM-5 with a crystal size less than 0.1 micron synthesized accordingto the method described in U.S. Pat. No. 3,926,782.

4. ZSM-5 with a crystal size less than 0.1 micron synthesized accordingto the method described in U.S. Pat. No. 5,369,071.

It will be seen from FIG. 1 that in the P/P₀ range of 0.4-0.7, theisotherm for the ZSM-5 of Example 1 had a slope 88.5 as compared withvalues for the conventional materials of 9.4 (material 1 above), 12.3(material 2 above), 23.5 (material 3 above) and 25.8 (material 4 above).

EXAMPLE 2

A synthesis mixture was prepared from 250 g of water, 80 g of amorphoussilica-alumina having a silica/alumina molar ratio of 25:1 (as suppliedby WR Grace), 4 g of sodium aluminate solution (19.5% Na₂O, 25.5%Al₂O₃), 10 g of 50% NaOH solution and 3 g of ZSM-5 seeds. The mixturewas reacted at 160° C. (320° F.) in a Parr autoclave with stirring at100 RPM for 168 hr (with interruptions for sampling). The product wasfiltered, washed with water and dried at 120° C. (250° F.).

The as-synthesized material was converted into the hydrogen form bymultiple ion exchange with ammonium chloride solution at 82° C. (180°F.), followed by drying in the oven and calcination at 540° C. in airfor 6 hours. The resultant HZSM-5 was found to have a silica/aluminamolar ratio of 19:1, an Alpha Value of 1750, a BET surface area of 412m²/g (364 zeolite surface area and 48 matrix surface area), and a D/r²parameter for 2,2-dimethylbenzene of about 2500×10⁻⁶.

EXAMPLE 3

A synthesis mixture was prepared from 250 g of water, 80 g of amorphoussilica-alumina having a silica/alumina molar ratio of 25:1 (as suppliedby WRGrace), 6 g of sodium aluminate solution (19.5% Na₂O, 25.5% Al₂O₃),10 g of 50% NaOH solution and 3 g of ZSM-5 seeds. The mixture wasreacted at 160° C. (320° F.) in a Parr autoclave with stirring at 100RPM for 192 hr (with interruptions for sample withdrawal). The productwas filtered, washed with water and dried at 120° C. (250° F.).

The as-synthesized material was converted into the hydrogen form as inthe previous Examples and the resultant HZSM-5 had a silica/aluminamolar ratio of 19:1, an Alpha Value of 1528 and a BET surface area of398 m²/g (346 zeolite surface area and 53 matrix surface area).

EXAMPLE 4

The HZSM-5 of Example 1 (Catalyst C in Table 1 below) was pelletized,crushed and sized to 12/40 mesh and then used to effect liquid phaseisomerization of a mixed xylene feed having the composition given inTable 1 below. For comparison, the same feed was isomerized usingcatalysts prepared from the conventional ZSM-5 materials (3) and (2) inExample 1 and referred to as Catalysts A and B respectively in Table 1.

In each case testing was conducted in an automated unit with on-linesampling using a 0.375 inch (0.95 cm) diameter stainless steel tubereactor into which was loaded 1 gm of the catalyst with sand as an inertpacking material. The reactor was pressurized with nitrogen to 550 psig(3890 kPa) and heated to 560° F. (293° C.) under flowing nitrogen. Afterintroduction of the liquid feed to the reactor, all gas flow wasstopped. Conditions for these runs were 8.8 WHSV, 550 psig (3890 kPa),and 0 H2/HC. Temperature was varied to effect xylene isomerization.Results are tabulated below.

TABLE 1 CATALYST Feed Catalyst A Catalyst B Catalyst C Temperature ° F.(° C.) 563 (295) 563 (295) 491 (255) Yields (wt %) Toluene 0.6 0.9 1.10.8 Ethylbenzene 0.3 0.3 0.3 0.3 Total Xylenes 98.4 97.8 97.6 98.2 TotalC9+ 0.7 1.0 1.1 0.7 Xylene loss, wt % 0.5 0.8 0.2 P-xylene/Total Xylene,wt % 12.6 23.4 23.1 23.3 Para approach to equilibrium, 95.0 92.6 92.5 wt%

The results given in Table 1 show that the ZSM-5 of the invention(Catalyst C) was 70° F. (40° C.) more active than either the highactivity preparation (Catalyst B) or the “small crystal” preparation(catalyst A). Further, utilizing the catalyst of the present inventionresults in lower xylene loss.

EXAMPLE 5

Catalyst C of Example 4 was used to effect disproportionation of atoluene feed at conditions including a temperature of 800° F. (427° C.),a pressure of 400 psig (2860 kPa), WHSV of 9.2 hr⁻¹ and a H2/HC (molar)of 1. The product yields were as follows:

C5− 1.0 wt % Benzene 19.5 wt % Ethylbenzene 0.5 wt % P-xylene 5.7 wt %M-xylene 12.4 wt % O-xylene 5.5 wt % C9+ 3.0 wt % Para Selectivity 24.2Toluene conversion 48 Benzene/xylene (molar) 1.12

What is claimed is:
 1. A synthetic porous crystalline material havingthe structure of ZSM-5 and a composition involving the molarrelationship: X₂O₃:(n)YO₂, wherein X is a trivalent element; Y is atetravalent element; and n is less than 20, and wherein the slope of thenitrogen sorption isotherm of the material at a partial pressure ofnitrogen of 0.4 to 0.7 and a temperature of 77° K is greater than
 30. 2.The crystalline material of claim 1 wherein the slope of the nitrogensorption isotherm at said partial pressure of nitrogen of 0.4 to 0.7 andtemperature of 77° K is greater than
 50. 3. The crystalline material ofclaim 1 wherein n is about 15 to about
 20. 4. The crystalline materialof claim 1 wherein the material has a mesoporous surface area (MSA)greater than 45 m²/g.
 5. The crystalline material of claim 4 wherein theratio of the zeolite surface area (ZSA) to MSA of the material is lessthan
 7. 6. The crystalline material of claim 1 wherein the material hasan Alpha Value in excess of
 1300. 7. The crystalline material of claim 1wherein X is aluminum and Y is silicon.